Particular embodiments generally relate to ground shield capacitors.
Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
For a passive component, such as an inductor or transformer, the area under the passive component in an integrated circuit (IC) chip is often left unused. This avoids the impact of the passive component on circuits under the passive component and the impact of the circuit on the passive component. The impacts include electric coupling (capacitive) and magnetic coupling (eddy currents).
A ground shield may be placed under the passive component to terminate electric fields resulting from electric coupling. Additionally, the performance of the passive component may be improved by the use of the ground shield. For example, the ground shield may increase an inductor's quality factor (Q). Also, the electric coupling between the passive component and a substrate or another structure under the passive component may be reduced. However, it is possible that ground shields will not block eddy currents, and thus, even when a ground shield is used, circuits are often not placed under the passive component.
Not having anything under the passive component may cause problems in chip fabrication. For example, it is better for chip fabrication to maintain the density of each metal layer between an upper limit and a lower limit. A passive component made with a high-level metal and nothing under the high-level metal layer violates density rules for lower-level metal. Workarounds exist that place metal fill around the passive component. However, the fill takes up additional area. Using a ground shield on a metal layer under the passive component will meet metal density rules without a guard ring of metal fill.
FIG. 1 shows an example of a transformer 102 with a conventional ground shield 104 for an integrated circuit (IC) chip. Although transformer 102 is shown, another passive component may be used. Transformer 102 in this example includes two coils, a primary coil and a secondary coil.
Ground shield 104 is situated under transformer 102 and includes a plurality of fingers 106. Fingers 106 include gaps in between them that do not allow a circle of current to flow around ground shield 104, which avoids the adverse effects of eddy currents.
Each finger 106 is coupled to contacts 108. This couples the fingers to a ground 110. Also, fingers 106 are all coupled to the same layers of metal.
In addition to ground shield 104, the chip may include a de-coupling capacitor. In some radio frequency circuits, a high frequency current is pulled from the supply. Bond wire inductance acts as a large impedance at high frequencies. So, an alternating current (AC) low impedance path to ground is required on the chip. Typically, a large de-coupling capacitor between supply and ground is used. These de-coupling capacitors require significant area on the chip.
One example of a de-coupling capacitor that may be used is a metal-oxide-metal (MOM) capacitor. FIG. 2 shows an example of a conventional MOM capacitor 200. MOM capacitor 200 includes a plurality of metal lines 202. Odd metal lines 202a may be connected to a first connection at the bottom, which may be connected to ground 204. Even metal lines 202b may be connected to a second connection at the top, which may be connected to a supply 206. Odd metal lines 202a and even metal lines 202b alternate in MOM capacitor 200. Capacitance between even metal lines 202b and odd metal lines 202a is then formed.
Conventionally, ground shield 104 and MOM capacitor 200 are separate structures in different areas of the chip. Having separate structures may be an inefficient use of area on the chip.